Method of recovering durene by crystallization and purifying it by washing with a solvent at low temperature



United States Patent METHOD OF RECOVERING DURENE BY CRYS- TALLIZATION AND PURIFYING IT BY WASH- lNG WITH A SOLVENT AT LOW TEMPERATURE Russell W. Walker, Lansing, 111., assignor to Sinclair Refining Company, New York, N.Y., a corporation of Maine 7 No Drawing. Application February 9, 1955 Serial No. 487,208

6 Claims. (Cl. 260-674) This invention relates to a method of recovering durene from a hydrocarbon mixture containing it. More particularly, the invention is directed to obtaining good yields of durene of a higher degree of purity than in the feed through crystallization and washing of the resulting crystals.

It has been the practice to separate durene from durene-containing hydrocarbon mixtures, such as naphtha reformate fractions boiling in the range between about 350 and 425 F. by crystallization, since separation of durene by distillation is generally impracticable because of the close proximity of the boiling point of durene to those of tetramethyl benzenes isomeric to durene, such as isodurene, usually present. in such mixtures. Durene and isodurene reportedly form a eutectic mixture at about --18 F., the composition of the eutectic mixture being about 92 percent isodurene and 8 percent durene. Processes for crystallizing durene have been devised to avert the formation of a eutectic mixture containing durene by cooling durene-containing hydrocarbon fractions to just short of the point Where the first eutectic of durene with accompanying compounds is formed. Such processes necessarily limit the yield of durene. Other processes involve repeated crystallization to obtain a durene product of greater purity. However, besides being costly, obtaining a relatively pure product by recrystallization is difficult, since durene crystals are small and their large surface area occludes excessive amounts of mother liquor,

Surprisingly, particularly in view of the prior teachings, I have found in the present invention that by the use of certain temperatures, substantially all of the durene in aromatic hydrocarbon fractions boiling within the range of about 360 to 400 F. and containing about to 30 percent durene can be crystallized with apparently no formation of a eutectic mixture. The absence of a eutectic is indicated by the phase behavior of the crystals as illustrated on a time-temperature plot of the crystallization operation. Possibly the eutectic is not formed because of the presence of hydrocarbons other than durene and isodurene in the aromatic feed. The relatively narrow boiling range of the feed also may exclude other eutectic-forming hydrocarbons.

In addition to the excellent yields of product that are obtainable in a single crystallization by the process of my invention, the product is readily and easily purified and if desired, to the exceptionally high degree of purity of at least 98 mol percent, by washing the product at a temperature below about 40 F. with a miscible, non-aromatic extractive solvent.

In the practice of the present invention, an aromatic hydrocarbon feed boiling in the range between about 360 to 400 F. and containing from about 5 to 30 weight percent of durene is cooled to below about ---40 tallization of about percent or greater of the amount of durene'originally present in the hydrocarbon feed.

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The durene crystals are then separated from the cooled fraction, and the crystals are washed at a temperature below about -40 F. with a suitable extractive material. Separation of any traces of extractive material from the crystals can then be effected in any suitable manner, if necessary.

The present invention has application to the separation of durene from reformates boiling in the range from about 360 to 400 F. and those aromatic hydrocarbon fractions boiling in this range obtained by distillation of crude petroleum, catalytic or thermal cracking and from other operations producing aromatic concentrates. A preferred crystal-lizer feed is a naphtha reformate fraction boling in the range between about 370 and 400 F. Advantageously, the fraction can contain about percent or more of aromatics, including about 15 percent of durene.

The temperature to which an aromatic feed stock must be cooled to crystallize a substantial portion, i.e., 90 percent or more of durene therein, will depend upon the concentration of durene in such stock. The lower the concentration of durene in the feed, the lower will be the temperature required to crystallize a substantial portion of the durene.. With the feed stocks of the present invention, I cool to a temperature below about 40 F. to insurethe crystallization of 90 percent or more of durene; Since yield of durene depends upon the amount of durene that is crystallized in the crystallization step, the feed is preferably cooled until substantially all of the durene in the feed is crystallized. Rate of cooling and holding time at the crystallization temperature do not aifect the yield or purity of the durene product to any significant extent, supercooling of durene not being a particular problem because of the relatively low crystallia zation temperatures employed.

Durene crystals are separated from the cooled fraction or mother liquor by centrifugation, filtration or other suitable methods. This separation step is preferably conducted at the crystallization temperature or only slightly thereabove, to prevent loss of durene to the solubilizing mother liquor. V

I have found that the mother liquor occluded on the surface of the durene crystals can be removed by washing the separated crystal mass with certain selective solvents or extractive materials at temperatures below about 40 F. The utilization of these low temperatures avoids losses of durene to the mother liquors. The extractive materials of this invention were also found to have a highly selective solvent action at temperatures below about 40 F., for at such temperatures they have substantially no solvency for durene. However, the solubility of durene in the extractive materials increases with increasing temperatures above about 40 F. Thus, washing thedurene crystals above. about -40 F. results in the loss of an appreciable amount of durene to the solvents. Preferably, the durene crystals are washed at the crystallization temperature or slightly thereabove substantially pure durene can be obtained by this pro cedure without any significant decrease in yield of prod- The extractive materials I employ are non-aromatic organic solvents containing up to about 8 carbon atoms which are miscible with the crystallization mother liquors at the wash temperature. Suitable non-aromatic extractive solvents include lower aliphatic hydrocarbons such as paraifins, olefins, alcohols, ketones, ethers, esters, and

cyclic hydrocarbons such as cycloparaffins. For example, among the paraflins, olefinsand cycloparafiins are those containing 3 to 8 carbon atoms such as propane, butane, pentane, propen'e and-cyclcpentane. Among suitable aliphatic alcohols are those containing. 1-to-5 carbon atoms such as methyl alcohol, and isopropanol. Aliphatic ketones containing 3 to6 carbon atoms, such as acetone and methyl isobutyl ketone, are examples efsuitable ketones. Also, I can use lower saturated and unsaturated chlorinated aliphatic hydrocarbons such as olefins and paraflins, e.g., ethylene dichloride, methyl chloride, carbon tetrachloride, ethyl chloride, trichloroethylene, and tetrachlo roethylene. Among useful aliphatic esters are those con raining-,4 to, 7 carbon atoms such as amyl acetate, isopropyl acetate, and ethyl acetate. Typical of the ethers and nitriles I employ are ethyl ether and acetonitrile. Mixtures of these solvents can also be employed, and successive washes with different solvents also are effective.

The preferred solvent, however, is propane.

The quantity of extractive material required to remove mother liquor absorbed or occluded on the durene crystals depends, to a large extent, upon the degree of purity desired and the particular solvent utilized. I prefer, however, that washing be continued until the crystal mass is at least about 98 mol percent durene. The durene crystals can be washed with solvent in batchwise or continuous fashion with recycle of the mother liquor when advantageous. Table I sets forth the results obtained by washing durene crystals from feeds containing about 10.8 percent and 19.0 percent durene with several solvents. The crystals were washed in batchwise manner, and the volume of solvent utilized for each Wash was about equal to the volume of the durene crystals in each instance.

Durene cake from feed containing about 19 percent durene crystallized and washed at temperature of 60 C.

2 Durene cake from feed containing about 10.8 percent durene crystallizcd and washed at temperature of 70 C.

Table I indicates that isopropanol has highly selective solvent properties, thus a lesser quantity of this solvent is required to obtain a purity of 99 wt. percent or higher, four volumes of isopropanol per volume of crystals being generally obtainable from most of my feeds if the process of the present invention is carried out in its preferred form, i.e., cooling the feed to a temperature below about 40 F. to crystallize substantially all of the durene, separating the cooled fraction or mother liquor from the crystals at the crystallization temperature, and washing the durene cake at below about -40 F'. with the volume required to obtain a product having a purity of at least 98 mol percent of a highly selective solvent such as propane or isopropanol.

A small amount of solvent may. be adsorbed or entrained by the durene crystals during the washing operation. Separation of solvent from the crystals can readily and easily be effected by distillation or by blowing the crystals with an inert gas such as air. The latter procedure is usually conducted at ambient temperature although it may be carried out at lower temperatures. S01- vents which readily adapt themselves for removal from crystals are the lower members of the paraffin series, such as propane, which are gases at room temperature and are vaporized from the crystals as they are warmed to such temperature. Solvent separation and refrigeration costs are apreciably decreased by the use of such solvents.

The following specific examples of the process of the present invention are given merely by way of illustration and are in no manner to be considered as limiting the scope of the invention.

EXAMPLE I A fraction boiling in the range between 370 and 397 F. and analyzing 19.0 percent by weight of durene was obtained by fractional distillation of a Mid-Continent reformate in a fractionating column rated at approximately 40 plates. A 28.3 g. aliquot of this fraction was cooled while stirring to 76 F. The resulting slurry was maintained at thecrystallization temperature for about 0.25 hour and then transferred to a Biichner type funnel which had been previously solvent-cooled to 76 F. 11.5 g. of mother liquor containing less than 1 percent durene was filtered off at 76 F. leaving 16.8 g. of a solid cake which analyzed 32 percent durene. The impure durene cake was washed with a total of 35 cc. of propane at 76 F. in four successive batchwise steps. The purified durene crystals were warmed to about 77" F. to vaporize residual traces of propane. 4.4 g., representing a yield of 81.2 percent yield of durene having a purity of greater than 99 mol percent were recovered.

Table 11 sets forth the results obtained utilizing various solvents in experimental procedures similar to that described in detail in Example I. In each of the tabulated examples, the purified durene was air dried as the filter and its contents were warmed to 77 F. to remove residual traces of solvent. Feed stocks were obtained from Mid- Continent reformates as indicated in Example I.

TABLE II Run 1 2 3 4 5 6 7 Feed:

B. P. range, "F 361-397.--- 361-397-. 383-391 370-397 37 .3

Wt. Percent Dureue.. 10.8 10.8- .0 19.0 19 19.0 2

Grams 53.3 35.0--. 32.1. 41.5 Crystallization and wash Tempera- -1ns -10s ss 24.0 23 37 Solvent Acetone-.- Trichloroethylene.

Total Wash, Volume, re fin Product:

Durene, g

Yield, Wt. Percent Minimum purity, mol. per

'Washed with 30 cc. pentane at 108 F., then with 5 cc. MeOH. at +77 F.

It is claimed:

1. The method of separating durene from a hydrocar- Durene yields. of at leastaboutSO wcight percent are bon feed containing about 5 to 30 percent durene,;iso-

durene and other aromatic hydrocarbons and boiling in the range between about 360 and 400 F. which comprises cooling the durene-containing fraction to a temperature below about 40 F. to crystallize at least about 90% of the durene in said fraction, separating durene crystals formed at a temperature below about -40 F. from the mother liquor, and washing said crystals at a temperature below about 40 F; with a non-aromatic extractive solvent containing up to about 8 carbon atoms which is miscible with entrained mother liquor at the wash temperature.

2. The method of claim 1 in which washing is conducted at about the crystallization temperature.

3. The method of separating durene from a hydrocarbon feed containing about 5 to 30 percent durene, isodurene and other aromatic hydrocarbons and boiling in the range between about 370 and 400 F. which comprises cooling the durene-containing fraction to a crystallization temperature below about 40 F. to crystallize substantially all of the durene in said fraction, separating durene crystals, formed at a temperature below about 40 F. from the mother liquor at about the crysta1lization temperature, and washing said crystals at a temperature below about 40 F. with a non-aromatic extractive solvent containing up to about 8 carbon atoms which 6. The method of claim 1 in which the durene-containing feed is a naphtha reformate fraction.

References Cited in the file of this patent UNITED STATES PATENTS 2,383,174 Weir Aug. 21, 1945 2,562,068 Souders July 24, 1951 2,625,555 Miller Jan. 13, 1953 2,665,316 Bennett Jan. 8, 1954 2,672,487 Teg-ge et a1 Mar. 16, 1954 2,766,310 Bennett et a1 Oct. 9, 1956 2,823,241 Bennett etal. Feb. 11, 1958 OTHER REFERENCES Smith et al.: Journal American Chemical Society,

5 volume 51, pages 3001-3008 (1929), 

1. THE METHOD OF SEPARATING DURENE FROM A HYDROCARBON FEED CONTAINING ABOUT 5 TO 30 PERCENT DURENE, ISODURENE AND OTHER AROMATIC HYDROCARBONS AND BOILING IN THE RANGE BETWEEN ABOUT 360* AND 400* F. WHICH COMPRISES COOLING THE DURENE-CONTAINING FRACTION TO A TEMPERATURE BELOW ABOUT ABOUT -40* F. TO CRYSTALLIZE AT LEAST ABOUT 90% OF THE DURENE IN SAID FRACTION, SEPARATING DURENE CRYSTALS FORMED AT A TEMPERATURE BELOW ABOUT -40* F. FROM THE MOTHER LIQUOR, AND WASHING SAID CRYSTALS AT A TEMPERATURE BELOW ABOUT -40* F. WITH A NON-AROMATIC EXTRACTIVE SOLVENT CONTAINING UP TO ABOUT 8 CARBON ATOMS WHICH IS MISCIBLE WITH ENTRAINED MOTHER LIQUOR AT THE WASH TEMPERATURE. 